Answer to Question #3034 Submitted to "Ask the Experts"

Q

I note that previous responses to questions on your Web site have explored radiation dose delivery to patients as part of medical examinations. I note that your experts' responses have not clearly defined the dose equivalence received for common extremity imaging. Can one of your experts provide an estimation which suggests the likely radiation dose received by a patient undergoing a "routine" knee CT examination (either as a true dose value or as a comparative figure)? In my wider personal reading into radiation safety, I have seen the line "the risk of radiation carcinogenesis based on the linear no-threshold approach" which, I presume, the authors are paraphrasing from a publication from an authoritative radiation safety body when discussing radiation risk to patients. I am unclear as to what this "approach" is and have been unable to find independent reference to it. Does one of the experts have a reliable, citeable value for the dose delivery of such a procedure and can they explain the approach mentioned above?

A

I will answer your questions in several separate parts, including the terminology used, as there are several terms that are appear to mean the same thing.

First of all, the dose equivalent (H) is different from the equivalent dose (H_T), which leads to problems of interpretations and discussions of exposures and doses. H is defined as the absorbed dose at a point in tissue weighted by the linear energy transferred to the tissue at that point. It is directly related to the energy deposited at the site of irradiation. On the other hand, the H_T is the average absorbed dose in the tissue or organ weighted by a radiation quality factor. The H_T is the term considered when we consider the risk to the entire organ. Finally, there is a term called the effective dose (E) which is defined as the sum of the equivalent doses, H_T, to each organ times a tissue weighting factor, wT, for that organ. The tissue weighting factor is based on the detrimental response of that tissue if the body was irradiated uniformly. That is, if the entire body was irradiated, what would be the chance of cancer occurring in the thyroid, lungs, etc.

Generally, one does not consider risk to extremity imaging or exposures, as there are few "critical organs" associated with the extremities. This is not to say that large doses of radiation cannot lead to adverse effects to the extremities, such as radiation burns that may occur during radiation accidents in industrial operations. However, large doses and effects are rare in imaging or diagnostic radiology. The equivalent doses, H_T, required for such effects, for example, 3 to 5 Gy (300 to 500 rad), far exceed the typical exposures delivered in diagnostic imaging. For CT exams, effective doses are typically around 0.01 Sv (1 rem or 1,000 mrem) For example, see the answer to Ask the Expert Q708.

As for your question about the CT dose to a knee examination, there are probably few individual studies done as they are not routinely performed compared to head or whole-body CT scans. Again, the risks of exposures to the extremities are negligible. For plane films, the effective doses, E, are about 0.06 mSv or 0.006 rem, which is significantly less than the effective doses that result from CT examinations of the head or body cited above.

Finally, you asked about the risk of radiation-induced carcinogenesis, based on the linear no-threshold approach or concept when discussing radiation risk to patients. The concept is really associated with exposures to radiation workers and members of the public. The idea is that we know that radiation exposure at high doses and dose rates results in increases in cancer. See the RERF Web site. I like to quote generally accepted values where the cancer incident rate increases in a large population by 0.05% per 0.01 Sv (1.0 rem). It is important to remember that in large populations cancers do occur naturally and that the estimated rate is frequently quoted as 25%. That is, for a population of 10,000 people, 2,500 may be diagnosed with cancer. It should be kept in mind that no estimate can be given for any single individual in the group. Thus, if each person in the 10,000 population receives 0.01 Sv of radiation, it can be estimated that there will be an additional 5 cancers, for a total of 2,505 cancers in the 10,000.

However, it is difficult to determine what the radiation risks are at low doses and dose rates. For regulatory purposes, it is assumed that at zero exposure there is zero risk. Thus, one can draw a linear relationship from a known effect at some dose to zero effect at zero dose. One could then say that for a 0.001 Sv exposure the risk of cancer increases by 0.005%, which is one-tenth of the value in the example above. The problem is that there is a natural, nonzero incidence of cancer in the population. One is confronted with the question whether changes in the cancer rates are due to radiation exposure or to natural fluctuations in the background cancer incident rate. The issue of the linear no-threshold hypothesis is controversial, with the Health Physics Society not taking a specific stand on the issue. See ATE Q413.

For medical exposures, the risk to patients must be based on the benefits derived from the radiation exposure received to either arrive at a diagnosis or treat a known medical condition or injury. For example, does the 0.01 Sv effective dose a patient receives when getting a head CT scan, which may increase the person's risk of developing cancer by 0.05%, outweigh the risk of not diagnosing a head injury which could lead to a stroke or death?

John Jacobus, MS, CHP